LED is a solid state semiconductor device which can directly convert electricity into light. The intrinsic characteristic of the LED decides that it is the ideal substitute for traditional light source, and therefore it has broad applications.
In addition to meeting safety requirements, the LED driving circuit should has two basic functions: firstly, it should maintain the constant-current characteristic as far as possible, especially when a change of ±15% occurs in the power supply voltage, it should be able to ensure that the change in the output current is within ±10%; secondly, the driving circuit itself should maintain a lower power consumption, so as to ensure that the LED system efficiency maintains at a high level.
In the prior art, as for the application of the multi-path constant-current LED driver, there are two common solutions:
1. a constant-voltage power supply together with multiple non-isolated DC/DC constant-current control circuits, such as, Buck circuits, i.e. Buck type conversion circuits.
As shown in FIG. 1, the output of the constant-voltage power supply is used as the input for each of the multiple constant-current circuits, and constant-current control is performed for each DC/DC conversion circuit independently, therefore constant-current driving of multiple LED branches may be easily ensured and meanwhile the efficiency of the driver is higher. When respective DC/DC constant-current control circuits together with respective LED branches constitute multiple loads of the constant-voltage power supply, the constant-voltage power supply in the front stage needs only two output wires to be connected to the multiple loads in the post stage, which means a simple wiring.
However, this solution has the disadvantage that the multi-path DC/DC constant-current control converter circuit is complicated and has high cost.
2. Output voltage adjustable voltage source together with multiple current-limiting circuits for circuitry adjustment.
As shown in FIG. 2, linear adjustment is performed by regulating MOS transistors Q1, Q2, . . . , Qn to achieve the constant-current driving of multiple LED branches. An output voltage adjustable voltage source 201 in the front stage samples a minimum drain voltage of the regulating transistor in the multiple linear adjustment current-limiting circuits 203 in the post-stage through a minimum value sampling circuit 202, and performs feedback control based on the minimum value by means of an output voltage control circuit 204, maintaining the minimum value at a low voltage value, so that the output voltage Vo of the output voltage adjustable voltage source 201 is always slightly higher than the voltage of one of the multiple branches which has the highest voltage, and the power consumption of the linear regulator circuit 203 is always close to the minimum value while ensuring that each LED load is driven at a current-limiting point.
In such a solution, the linear adjustment current-limiting circuit in each branch has low cost, and may achieve relatively high efficiency when the voltage difference between the multiple branches of LEDs is smaller. However, the solution has the following disadvantages: the output voltage control circuit 204 of the output voltage adjustable voltage source 201 in the front stage needs to sample the voltage in the post-stage circuit formed by the branch of LED(s) and the linear adjustment current-limiting circuit 203, therefore the wiring between the front-stage output voltage adjustable voltage source 201 and the post-stage circuits is complicated. Moreover, in this solution, for the convenience of the output voltage adjustable voltage source to perform the voltage sampling from the post-stage circuit, the linear adjustment current-limiting circuit 203 is typically required to be arranged inside the driver together with the output voltage adjustable voltage source 201, therefore the heat generation of the driver is tremendous for the reason that the consumption of the regulating transistor is large when the voltage difference between the multiple branches of LED(s) is large, which affects the life time and reliability of the driver; when a open-circuit failure occurs in a LED branch, the voltage on the drain of the linear regulating transistor in this LED branch is zero, therefore additional open-circuit protection is required in order to maintain the normal operations of other LED branches.